The present invention relates to a hydraulic circuit for the hydrostatic transmission of a vehicle, said hydraulic circuit comprising at least one hydraulic motor, two main ducts, serving respectively as feed and as discharge main ducts for the motor, a parking brake comprising brake means suitable for being urged by return means into a braking position in which they brake the motor, and for being urged into an inactive position by feeding fluid to a brake release chamber via a brake release duct, the circuit further comprising brake release control means suitable for connecting said brake release duct to a first brake release fluid source, means for connecting the brake release chamber to a unpressurized reservoir, short-circuit valve means suitable for isolating said feed and discharge main ducts from each other or for interconnecting them, selection valve means suitable for connecting the brake release duct to the brake release chamber or for isolating said duct from said chamber, and auxiliary brake release means suitable for connecting the brake release chamber to a second brake release fluid source when said chamber is isolated from the brake release duct.
When the motor is operational for driving the vehicle, brake release is achieved by feeding fluid to the brake release chamber, so that the brake means take up their inactive position. To achieve mechanical braking, it is necessary to cease feeding the brake release duct from the first brake release fluid source, and to connect the brake release chamber to the unpressurized reservoir, so as to enable said chamber to be emptied.
For example, a valve is disposed on the brake release duct and connects said duct either to the first brake release fluid source or to the unpressurized reservoir.
When the circuit is not fed with fluid, the motor is inactive, and the brake release chamber is not fed, so that the brake means are in their braking position under the effect of the return means which are, in particular, resilient return means such as a spring.
It is sometimes necessary to tow the vehicle. In order for towing to take place properly, two conditions must be satisfied. Firstly, it is necessary to prevent the hydraulic motor from delivering torque that opposes the towing, i.e. the motor must be deactivated by enabling its rotor to turn in freewheel mode. For this purpose, the short-circuit valve means are controlled so that they interconnect the feed and discharge main ducts, i.e. so as to form a short-circuit between said ducts.
The second condition is that the brake must be deactivated. The first brake release fluid source can be an auxiliary pump such as a booster pump or the main pump of the circuit, which pump is driven so as to deliver the fluid under pressure when the vehicle is operating, in general by means of an engine. For towing purposes, the first brake release fluid source is not used, in particular because said engine is not in operation. It is therefore the second brake release fluid source that is used to deactivate the brake for towing purposes. For example, said second source can be a handpump, a pump fed by a small independent motor, or else a pressurized fluid source available on the vehicle for a function other than driving it (e.g. for powering a tool), even when the transmission of the vehicle is not operational. To achieve brake release for towing purposes, the selection valve means isolate the brake release duct from the brake release chamber which must be connected to the second brake release fluid source.
U.S. Pat. No. 4,317,331 shows a hydraulic circuit of this type, in which the short-circuit valve means and the selection valve means are constituted by two distinct hydraulic entities, each having its own specific control means.
An object of the present invention is to provide a simplified hydraulic circuit, making it possible to provide the functions of the short-circuit valve and of the selection valve simply, reliably, and inexpensively.
This object is achieved by the fact that the circuit of the invention has a single short-circuit and selection valve having two main ports connected to respective ones of the two main ducts, and two selection ports connected respectively to the brake release chamber and to the brake release duct, and control means for controlling said valve, which control means are suitable for causing the valve to take up a first configuration in which the main ports are mutually isolated while the selection ports are interconnected, and for causing it to take up a second configuration in which the main ports are interconnected while the selection ports are mutually isolated.
The single short-circuit and selection valve of the invention makes it possible, in a single hydraulic element, to provide the function of the short-circuit valve means and the function of the selection valve means. As a result, the circuit is naturally more compact than in the prior art, and it is easier to put the valve in place in the circuit. In addition, it is much simpler to organize towing. In the first configuration, the hydraulic motor can operate since the main ducts are isolated from each other, and since, at the same time, the selection ports are interconnected so as to allow brake release to take place via the first brake release fluid source. Conversely, in the second configuration, towing is possible since the two main ducts are interconnected, i.e. the motor is short-circuited, and the selection ports are isolated so as to enable brake release to take place via the second brake release fluid source. For this purpose, it is necessary merely to connect said second source to the brake release chamber. To go from a situation in which the motor can operate to this situation in which towing is possible, only the hydraulic entity constituted by the short-circuit and selection valve needs to be controlled, whereas, in U.S. Pat. No. 4,317,331, two valves need to be controlled.
Advantageously, the auxiliary brake release means comprise an auxiliary brake release duct suitable for connecting the brake release chamber to the second brake release fluid source when the short-circuit and selection valve is in its second configuration.
In which case, the second configuration of the short-circuit and selection valve of the invention is used to enable brake release to take place by means of the second brake release source, via the auxiliary brake release duct. In other words, when the valve is in the second configuration, it is not necessary to actuate any hydraulic element other than the second brake release source (as appropriately connected to the auxiliary brake release duct) in order to achieve brake release.
In which case, advantageously, the short-circuit and selection valve further has an auxiliary brake release port suitable for being connected to the second brake release fluid source, and the auxiliary brake release duct is formed in said valve and is suitable, when said valve is in the second configuration, for causing the selection port that is connected to the brake release chamber to communicate with said auxiliary brake release port.
The auxiliary brake release port is then part of the short-circuit and selection valve of the invention, and it is thus to said port that the second brake release fluid source can be connected to achieve brake release for towing purposes, in which case the fluid is fed to the brake release chamber by means of the selection port which is connected thereto. It is also possible to consider having the auxiliary brake release port connected continuously to said selection port, but having said auxiliary brake release port closed off when towing is not necessary, e.g. by a stopper situated outside the valve.
Advantageously, the short-circuit and selection valve is suitable for taking up an intermediate configuration in which the main ports are interconnected, and in which the selection ports are interconnected and communicate with the auxiliary brake release duct.
This intermediate configuration is advantageous when the circuit is such that, when the motor is in the operating situation, brake release and mechanical braking are respectively obtained by causing the brake release duct to communicate respectively with the first brake release fluid source and with the unpressurized reservoir. In which case, when the motor is stopped, the brake release duct is normally put in communication with the unpressurized reservoir. It is thus in this situation that towing is made possible, by means of the single valve of the invention. Once towing is finished, the above-mentioned intermediate configuration makes it possible momentarily to put the auxiliary brake release duct in communication with the brake release duct, which is itself in communication with the unpressurized reservoir. As a result, the intermediate configuration makes it possible to empty the auxiliary brake release duct, and facilitates going from the brake release situation for towing purposes to a situation in which mechanical braking is possible again.
Advantageously, the control means for controlling the single short-circuit and selection valve are mechanical means.
The mechanical means may be of any known type for mechanically displacing an element between two positions, e.g. operating by a cam system, by a screw system, or by any other suitable means. Rather than having mechanical control means, it is however possible to use control systems operating by fluid pressure or any other known means for driving a moving member.
In a particularly advantageous configuration, the short-circuit and selection valve is disposed in a casing portion of the hydraulic motor that is provided with two main orifices connected to respective ones of the feed and discharge main ducts and to respective ones of the main ports of said valve, two auxiliary ducts, connected respectively to the brake release chamber and to the brake release duct and to respective ones of the selection ports of the valve, also being provided in said casing portion.
The casing portion of the hydraulic motor is machined to have a recess in which the valve of the invention can be disposed. It is also machined so that said recess is connected respectively to the main ducts, to the brake release chamber, and to the brake release duct. The valve constitutes a single hydraulic entity which is easy to put in place in its recess. The valve requires very little space, and the circuit is compact. In addition, the valve is easy to remove without it being necessary to disassemble a major portion of the motor.
Advantageously, the short-circuit and selection valve is provided with a return port connected to a unpressurized reservoir, and, when said valve is in the second configuration, the main ducts are connected to said return port.
In which case, during towing with the motor short-circuited by the valve being in the second configuration, so that motor operates in freewheel mode with its rotor being rotated and its pistons able to move, the cylinders whose pistons are moving outwards are prevented from being fed again. By being connected to the leakage return duct, the main ducts are substantially unpressurized.
In which case, advantageously, the return port is connected to a leakage return duct of the hydraulic motor, which duct is itself connected to the unpressurized reservoir.